Frets on fire:

Frets on Fire is a game of musical skill and fast fingers. The aim of the game is to play guitar with the keyboard as accurately as possible.
Frets on Fire is open-source and includes: a unique inverted keyboard gameplay style; support for guitar controllers and
generic joysticks; a song editor for making your own tunes; the ability to compete with others on the World Charts; hundreds of songs composed by the community; and support for importing Guitar Hero and Guitar Hero II songs.

download(pc version):
http://www.download.com/3001-7551_4-10661368.html

download(mac version):
http://sourceforge.net/project/downloading.php?groupname=fretsonfire&filename=FretsOnFire-1.2.451-macosx.zip&use_mirror=osdn

Enjoy …..ps. you can also hook up your 360 guitar hero guitar to your pc and use it to play.